Lubricating oils perform numerous functions. In engines they function to reduce friction and wear of numerous parts in moving contact with each other such as piston rings and cylinder walls, valves, cams, bearings, etc. They also function to cool the engine by operating as a heat sink and to clean the engine by carrying away combustion products and engine wear debris. Consequently engine oils accumulate various and numerous contaminants which are or can be harmful to the engine. Similarly, engine oils by being exposed to extremes of heat are subjected to oxidation which further increases the concentrations of various contaminants in the oil. The same can be said for lubricating oils other than engine oils. Transmission fluids, hydraulic oils, gear oils, turbine oils, functional fluids, industrial oils all function to lubricate parts in moving relationship with each other and in the course of their use accumulate various contaminants which are or can be harmful to the parts being lubricated.
One of the types of damage which can be caused by contaminants in lubricating oils which accumulate in the oils over time or which can even be caused by additives added to the oil to achieve some otherwise desirable end result, such as anti-wear additives, is corrosion of the ferrous and non-ferrous metal parts of the apparatus or device being lubricated. Metal corrosion, therefore can occur at any time in the course of using the lubricating oil. Corrosion can occur at the very beginning of the lubricating process if the oil contains an additive which, while desirable for some purpose such as antiwear or friction reduction, also by its very chemical make-up is corrosive to metal, or the oil can become corrosive over time as contaminants build up in the oils as a consequence of accumulation of combustion products (and their conversion into corrosive materials) or during lubricant aging, oxidation and normal deterioration.
Equipment manufacturers, as well as oil formulators, therefore, are faced with the dual problems of corrosion caused by additives as well as by contaminants. Finding a cure for such initial and long term metal corrosion problems, especially with the current impetus for extended oil drain intervals and sealed for life lubrication has fostered numerous solutions.
Lubricating oils are formulated to include anti corrosion additives. Surface active metal passivators are included into the lubricating oil to interact with the surfaces of the metal parts and render the metal resistant to the action of corrosive materials, be they other additives or accumulated contaminants, in the lubricating oils.
Anti-rust additives (corrosion inhibitors) protect the metal surfaces by preventing attack of the metal surfaces by water or other contaminants. Some anti-rust additives are polar compounds that wet the metal surface preferentially, protecting the metal surface with a hydrocarbonaceous-type or oil-type film. Other anti-rust/anti-corrosion additives absorb water by incorporating it into a water-in-oil emulsion so that only the continuous oil phase is in contact with the metal surface. Yet other anti-rust additives chemically adhere to the metal to produce a non-reactive surface. Suitable additives include zinc dithiophosphates, metal phenolates, basic metal sulfonates, fatty acids, amines.
Other metal corrosion inhibitors include thiadiazoles, e.g., dialkyl dimercapto thiadiazoles, triazoles, e.g., benzotriazole, toluoyltriazole. Such anti-corrosion materials are commonly incorporated into oils in an amount in the range of about 0.01 to 5.0 wt %, preferably about 0.01 to 1.5 wt %, more commonly, especially in the case of the surface active corrosion inhibitors, in an amount in the range of about 0.01 to 1.0 wt %.
U.S. Pat. No. 6,180,575 discloses high performance lubricating oils. The patent, in the Background of the Invention, indicates that the properties of oils may be differentiated on the basis of whether they are bulk properties which are not affected significantly by contact with the surface of other materials, or are surface-related properties which affect and are affected by the surfaces with which the oil is in contact. Oxidation resistance of the oil, for instance, belongs largely in the former category although the rate at which an oil undergoes oxidation in use is affected by the character of the metal surfaces in contact with the oil. Extreme pressure resistance may also be included in this category. Other properties such as anti-corrosion, anti-rust, anti-wear are directly dependent on the nature of the surfaces—usually metal—with which the oil is in contact during use. The properties which are surface dependent impart another consideration into the formulation of a finished lubricant since the additives which are used to improve the properties of the lubricant base stock and provide the desired balance of properties may be in competition for available sites on the metal surface. For this reason, it is often difficult to obtain a good balance between the performance properties which are surface dependent. One instance of this is with anti-wear and anti-rust properties: it is difficult to produce an oil which possesses both properties in good measure at the same time.
Different types of base stocks have different performance characteristics. Ester base stocks, for example, the neopentylpolyol esters such as the pentaerythritol esters of monobasic carboxylic acids, have excellent high performance properties as indicated by their common use in gas turbine lubricants. They also provide excellent anti-wear characteristics when conventional anti-wear additives are present and they do not have any adverse effect on the performance of rust inhibitors. On the other hand, esters have relatively poor hydrolytic stability, undergoing hydrolysis readily in the presence of water at even moderate temperatures. They are, therefore, less well suited for use in wet applications such as paper-making machinery.
Hydrolytic stability can be improved by the use of hydrocarbon base stocks. The use of alkyl aromatics in combination with the other hydrocarbon base stocks such as hydrogenated polyalphaolefin (PAO) synthetic hydrocarbons and the improved hydrolytic stability of these combinations is described, for example, in U.S. Pat. No. 5,602,086. Traditional formulations containing PAO's, however, present other performance problems. Although the hydrolytic stability of hydrocarbon base stocks including PAO's is superior to that of the esters, it is frequently difficult to obtain a good balance of the surface-related properties such as anti-wear and anti-rust because, as noted above, these surface-related properties are dependent upon the extent to which the additives present in the base stock compete for sites on the metal surfaces which they are intended to protect and high quality hydrocarbon base stocks such as PAO's do not favorably interact with the additives used for this purpose.
U.S. Pat. No. 6,180,575 addresses this problem by presenting a lubricating oil composition having improved anti-wear and anti-rust performance characteristics which comprises a base fluid which comprises at least 50 wt % of a hydrocarbon base fluid and an additive combination comprising (a) an adduct of a substituted triazole and a hydrocarbon amine phosphate in an amount below about 5 wt % of the total composition, and, (b) a trihydrocarbyl phosphate in an amount up to 5 wt % of the total composition wherein the ratio of the trihydrocarbyl phosphate to the adduct is between about 2:1 to about 5:1. The base fluid is characterized as a hydrocarbon oil of mineral oil origin or synthetic which is of lubricating viscosity, is saturated in character with a viscosity index of 110 or greater, has a sulfur content below 0.3 wt % and a total aromatics and olefinic content below 10 wt % each. The hydrocarbon base fluid comprising at least 50 wt % of the base fluid can comprise a hydroisomerized wax of mineral oil origin or a hydroisomerized Fischer-Tropsch wax or poly alpha olefin synthetic hydrocarbon, as well as base oils of mineral oil origin in Group II and Group III and other synthetic hydrocarbon stocks including those of Group V, and preferably the poly alpha olefins. The patent does not distinguish between these different base oils and only exemplifies mixtures of PAO and alkylated naphthalene.
U.S. Pat. No. 6,080,301 is directed to premium synthetic lubricant base stocks having at least 95% non-cyclic paraffins. Such base stock is produced by the hydroisomerization of Fischer-Tropsch synthesized wax under a particular set of conditions. The patent recites that this hydroisomerized stock and lube oils formulated from them have exhibited properties superior to PAO and conventional mineral oil derived base stocks. Consequently these hydroisomerized stocks can be used as is or blended into formulations in amounts constituting 20%, 40%, 60% or more of the base stock used in the formulations and can contain various additives or additives packages. The text also indicates that additive packages can and do contain many different chemical types of additives and the performance of the hydroisomerized base stock of the patent with a particular additive or additive package cannot be predicted a priority. In the patent the superiority of the unadditized hydroisomerized Fischer-Tropsch wax base stock over PAO and mineral oil stock is only demonstrated by superior resistance of the oil per se to oxidation and nitration.
U.S. Pat. No. 6,475,960 similarly is directed to premium synthetic lubricants which are derived waxy, paraffinic Fischer-Tropsch synthesized hydrocarbon by hydroisomerizing said stock, dewaxing and fractionation. It is indicated that the isomerized stock fraction boiling in the lube oil boiling range can be used as base stock in formulation typically containing one or more additives such as detergents, dispersants, antioxidants, anti-wear additives, pour point depressant, VI improvers, corrosion inhibitors such as benzotriazole and the like. In the examples of such isomerized stock containing an additive package, the additive package contained viscosity modifier PIBSA-PAM dispersant, detergents, antioxidants, ZDDP anti wear additive, demulsifier and anti foaming agent or it contained PIBSA-PAM, PIB-SA dispersants, antiwear additives, detergent, anti-oxidant, friction modifier, demulsifier and anti-foamant. These hydroisomerized Fischer-Tropsch wax derived base stock formulations were compared against formulations based on mineral oil and PAO using the same recited additive packages. The formulations were evaluated for panel coker deposits, oxidation stability and CCS viscosity wherein the formulations based on the isomerized Fischer-Tropsch wax exhibited their superiority in regard to each of these characteristics, based on the nature of the base oils, per se.
U.S. Pat. No. 6,191,078 relates to an aviation piston engine oil containing from about 60 to 75 wt % mineral oil base stocks, from about 15 to about 40 wt % polyalpha olefin (PAO), about 3 to 6 wt % PIBSA/PAM ashless dispersant, a viscosity index improver, a corrosion inhibitor, antiwear agent, a pour point depressant, an antifoam and an antioxidant.
U.S. Pat. No. 5,858,932 discloses a lubricating oil composition for internal combustion engines that contains A) 30-98% by weight of a mineral oil having a viscosity 2-30 mm2/s at 100° C. and a viscosity index not less than 100, B) 2-70% by weight of a polyalpha olefin, zinc dithiophosphate in an amount corresponding, as reduced amount of phosphorus, to 0.02-0.15 parts by weight with respect to 100 parts by weight of the base oil.
U.S. Pat. No. 6,060,437 relates to a lubricating oil composition for internal combustion engines that comprises A) a major amount of a base stock of lubricating viscosity containing from greater than 35 to less than 70 mass % of one or more PAO's, the balance preferably being one or more Group I base stocks and B) two or more additive components such as an ashless dispersant and a metal detergent.
WO 2004/003113 patent relates to a lubricant composition comprising a mixture of at least two Fischer-Tropsch derived base oils and one or more additives wherein one Fischer-Tropsch derived base oil (low viscosity component) has a kinematic viscosity at 100° C. of less than 7 mm2/s and a second Fischer-Tropsch derived base oil (high viscosity component) has a kinematic viscosity at 100° C. of more than 18 mm2/s.
US Publication 2004/0118744 relates to a lubricating base oil composition having at least 95 wt % saturates, of which saturates fraction between 10 and 30 wt % are cyclo-paraffins and the reminder being n- and iso-paraffins, having a viscosity index of above 120 and a pour point below −15° C.
According to Rudnick and Shubkin [“Poly (α-olefin)”, L. R. Rudnick and R. L. Shubkin, Chapter 1, in “Synthetic Lubricants and High-Performance Functional Fluids, 2nd Edition, L. R. Rudnick and R. L. Shubkin (ed.), Marcel Dekker Inc., New York, N.Y., 1999], poly (α-olefin), or poly alpha olefin (PAO), is a hydrocarbon manufactured by the catalytic oligomerization (polymerization to low molecular weight products) of linear olefins having six or more carbon atoms. Common commercial low-viscosity grades of PAO that are useful as lubricant base stocks have kinematic viscosity at 100° C. in the range of about 2-10 mm2/s, with viscosity index in the range of about 124-143.
According to R. A. Phillipps [“Highly Refined Mineral Oils”, Ronald A. Phillipps, Chapter 17, in “Synthetic Lubricants and High-Performance Functional Fluids, 2nd Edition, L. R. Rudnick and R. L. Shubkin (ed.), Marcel Dekker Inc., New York, N.Y., 1999.], base oils that are produced via hydrocracking and/or hydroisomerization processes are broadly referred to as VHVI (i.e., very high viscosity index) base oils. Common examples of VHVI base stocks have kinematic viscosity at 100° C. of about 4 mm2/s and about 6 mm2/s, with viscosity index in the range of 128-146. R. A. Phillipps cites Shell XHVI and Exxon Exxsyn as examples of VHVI base stocks and states: “It will be noted that Shell XHVI and Exxon Exxsyn are both virtually 100% saturates with no measurable aromatics.” Further, R. A. Phillipps states “As a point of interest, VHVI base oils can be manufactured by the synthesis of natural gas to wax, using the Fischer-Tropsch process. The wax can then be processed by hydroisomerization or catalytic isomerization of the wax to produce VHVI base oils. Evaluation of these VHVI base oils shows that they exhibit excellent oxidation stability as well as the very high VI shown by the laboratory tests. Performance is comparable to that obtained by poly (α-olefins), which might be expected because of their broadly similar chemical structure, but at reduced costs of production.”
GTL/wax isomerate base stocks and base oils of this invention have kinematic viscosity at 100° C. in the low-viscosity range of about 3-10 mm2/s, and in the higher-viscosity range of greater than 10 mm2/s, with viscosity index in the range of about 130 or greater, preferably about 135 or greater, and more preferably about 140 or greater. GTL/wax isomerate base stocks and base oils of this invention fit the definition of VHVI base oils, as cited by R. A. Phillips.
Both PAO and GTL/wax isomerate base stocks and base oils have virtually 100% saturates with nil aromatics.
Therefore, one expert opinion holds that GTL/wax isomerate base stocks and base oils would have exhibited performance comparable to that obtained by PAO base stocks and base oils.